Abstract:

Early theoretical models for the evolution of male-female
pairing were based largely on studies of birds. These models
assumed that biparental care of eggs and young was an
essential component of pairing. However, male-female pairing
is also a relatively common social system in coral-reef
fishes, and biparental care of young is extremely rare in
this group. Although pairing has been documented in at least
13 families of reef fish, surprisingly little is known about
the environmental and social factors that may maintain
heterosexual pairs as the basic social units. I tested two
hypotheses for pairing in a common Caribbean fish, the
cleaning goby Gobiosoma evelynae, a territorial species which
inhabits living coral heads. According to the Environmental
Constraints Hypothesis, pairing in G. evelynae is simply a
consequence of three related environmental parameters--low
male mobility, low female density, and a uniform distribution
of resources required by females. On reefs off St. Thomas,
U.S. Virgin Islands, I found positive size-assortative
pairing, frequent male movement between territories, rapid
re-pairing by both males and females after experimental
removal of a partner, and random distributions of apparently
suitable coral heads required by females for territory space.
These patterns are inconsistent with the Environmental
Constraints Hypothesis for pairing.
Mate Guarding is an alternative hypothesis for pairing and
is based on social interactions. This hypothesis, which
states that paired gobies maintain exclusive access to their
partners by expelling all potential sexual rivals, predicts
sex- and size-specific aggression toward conspecifics.
released large and small male and female gobies directly onto
the territories of pairs, and recorded the behavioral
responses of the resident fish. As predicted, residents
exhibited the greatest aggression toward large intruders of
the same sex, and the least aggression toward large intruders
of the opposite sex. Unpaired territorial females also
responded aggressively to experimentally added females, and
ignored added males. These results indicate that male-female
pairs are maintained by mutual intrasexual aggression in this
species, and that females (and possibly males) defend both
partners and other resources associated with their
territories.